1. Field of the Invention
This invention relates to a non-contact type tonometer in which a fluid for transfiguring a cornea is ejected toward the cornea in order to transfigure the cornea and the intraocular pressure is measured based on the transfiguration of the cornea, and more particularly to an improvement of an alignment control apparatus with respect to the cornea under test.
2. Description of the Related Art
Heretofore, a non-contact type tonometer is known as disclosed in, for example, a U.S. Pat. No. 3,756,073. This conventional non-contact type tonometer utilizes an air pulse as a fluid for transfiguring the cornea. The conventional non-contact type tonometer includes a fluid ejecting nozzle adapted to eject the air pulse. The axis of the fluid ejecting nozzle is arranged so as to be in alignment with an optical axis of an observation optical system for observing the cornea. When the optical axis and the corneal axis connecting the corneal vertex to the center of curvature of the cornea are brought into alignment with respect to each other, and the distance from the center of curvature of the cornea to the tip portion of the fluid ejecting nozzle is set to be in a predetermined distance, the fluid ejecting nozzle ejects the air pulse toward the cornea under test. The cornea under test is pressurized and transfigured flat by the air pulse. The deformation under pressure of the cornea is detected by monitoring system including a light transmitter adapted to project a collimated beam of light onto the cornea and a telescopic receiver adapted to receive a reflection light from the cornea. The non-contact type tonometer is such designed so as to measure the intraocular pressure based on a predetermined quantity of deformation of the cornea.
The related art disclosed in U.S. Pat. No. 3,756,073 includes an alignment verification system adapted to verify the alignment between the corneal axis and the optical axis of an observation optical system and also to verify the distance (hereinafter referred to "working distance") from the center of curvature of the cornea to the tip portion of the fluid ejecting nozzle. The alignment verification system for the conventional non-contact tonometer includes a target projection system adapted to project an image of the target toward the cornea. The alignment verification system disclosed in the U.S. Pat. No. 3,756,073 uses an objective lens of the observation optical system as the target projection system. In the alignment verification system, the light from the target is projected in such a manner as to form an image in the center of the curvature of the cornea through the objective lens, and reflected light due to specular reflection of the cornea is returned again to the observation optical system through the objective lens in order to reimage a target image on the aiming reticle. According to the conventional non-contact type tonometer, the alignment with respect to the cornea is effected according to the sharpness and the position of the target image on the aiming reticle.
However, the conventional alignment verification system has a problem that alignment with respect to the cornea can not be effected correctly, since only one target image is formed on the aiming reticle which makes it difficult to examine the sharpness of the image and the position thereof on the aiming reticle. Furthermore, the related art has such a disadvantage as that since the light from the target is projected in order to form the target image in the center of curvature of the cornea, the working distance is obliged to depend on the radius of curvature of the cornea of the eye under test. Thus, measuring errors are often resulted.